1. Field of the Invention
The present invention relates to a method and an apparatus for bending a pipe in a semicircle shape without using a circular tool for bending a pipe. The present invention especially relates to a method and an apparatus for bending a a pipe which are useful to work a corrugate tube.
2. Description of the Related Art
The conventional method for bending a pipe is, for example, shown in Japanese Unexamined Patent Publication No. 65419/1990 (Kokai). In this Publication, as shown in the side view of FIG. 7 and the A to A cross-sectional view of FIG. 8, by using a circular coma 2 which forms a processing groove whose outer peripheral surface is inserted by a pipe 5 and whose cross section is semicircle and an outer side arresting tool 4 which forms a processing groove whose bottom is inserted by the pipe 5 and whose cross section is semicircle, one end 1 of the pipe is fixed and at the same time, the the pipe 5 is inserted and fixed into the groove of the coma 2, the groove of the outer side arresting tool 4 and the pipe is rotated as arresting the pipe along the outer periphery of the coma 2 and the pipe is bent. In these figures, 1 denotes the pipe fixing side, 3 denotes the rotation center of outside arresting tool 4, 5 denotes the pipe moving side, 6 denotes the position of the outside arresting tool 4 in the state in which the pipe is bent at the angle of 90.degree., 7 denotes the pipe position in the state the pipe is bent at the angle of 90.degree., 8 denotes the position of the outside arresting tool 4 in the state in which the pipe is bent at the angle of 180.degree., 9 denotes the pipe position in the state in which the pipe is bent at the angle of 180.degree. and 10 denotes the rotation locus of the outside arresting tool 4.
In the conventional method for bending a pipe, it is very difficult to arrest the bent shape continuously in the case in which the cross sectional shape of the material is inhomogeneous (for example, a corrugate tube whose cross sectional shape varies in response to regions). Accordingly the conventional method for bending a pipe is difficult to apply to the above-mentioned material.
In this conventional method, the plastic deformation caused by bending depends on mainly the tensile deformation of the outer side material. Accordingly, the bent outside of the pipe material is extended with processing and the reduction in the thickness at that portion causes the problem especially when the internal pressure is effected on the bending product. Furthermore for the same reason, when the bending is conducted in the case of the radius being small, the outer peripheral portion is not extended to full length and it reaches to the inner peripheral surface and as the result, the ratio in which the bent cross section is compressed and the ratio of reducing area are increased.
There have been various proposals so as to control reduction of the local thickness of pipe wall which is generated at the time of bending a pipe. For example, in the invention for bending a pipe described in Japanese Unexamined Patent Publication No. 290622/1990 (Kokai), the proposal is described as follows. In this invention, a pipe is clamped by using a bending die which is rotatable, a pressure die which is affected by the bending reaction force when the pipe is clamped and bent by the tightening die which is able to revolve round this bending die and a clamping die which is opposite to this pressure die and which clamps the pipe; the pipe which is clamped by the pressure die and the clamping die is moved; and the compressive force is added to the axial direction of the pipe.
On the other hand, with respect to the bending of a corrugate tube, the shape deformation caused by the spreading toward the pipe's axial direction at the convex portion of the pipe occupies almost of the bending deformation. Accordingly, the bending is concentrated on the convex portion of the corrugate tube which was deformed at first and that portion is mainly compressed. Therefore it is impossible to give the desired bending to the corrugate tube.
In order to overcome the above-mentioned problem, a method for bending a corrugate pipe which is described in Japanese Unexamined Patent Publication No. 177261/1993 (Kokai) was proposed. In the invention of this Publication, by using a pipe arresting means provided with opposite surfaces which clamp the plurality of convex portions of the corrugate pipe toward the direction which is perpendicular to the pipe bending surface and the pipe center axis, bending a corrugate pipe is conducted. In this invention, that the bending and compressing are concentrated on a specific convex portion of the corrugate pipe is prevented from being generated and it becomes more easy to bend the convex portion of the corrugate pipe which is not deformed and that the bending of the corrugate pipe is concentrated on the limited part is prevented from being generated.
However in the above-mentioned invention of the apparatus for bending pipe described in Japanese Unexamined Patent Publication No. 290622/1990 (Kokai), compressive force toward the axial direction of the pipe is added at the time of bending a pipe. Therefore, when a corrugate tube is applied to this conventional invention, the corrugate tube may be compressed owing to the addition of the compressive force of the pipe. Accordingly, it is impossible to apply the apparatus for bending a pipe of this invention to a corrugate tube.
Also in the method for bending a pipe described in Japanese Unexamined Patent Publication No. 177261/1993 (Kokai), bending portion is clamped and arrested by the corrugate pipe arresting portion. Therefore, the following problem occurs in this invention: in addition to the fact that the deformation resistance is big, in order to obtain the accurate bending portion having the desired radius of curvature, rather large amount of skill and experience is needed.